Honey, climate change is shrinking the species

The old adage that bigger is better could be about to go out of fashion. Ecologists say climate change will shrink species.

But don't look out for hot shrinking animals just yet - the effects are likely not to be seen for many more years. Yet Kaustuv Roy, a biologist at the University of California in San Diego, believes we need to think now about how we are going to preserve large species.

The degradation of natural environments around the world is having the same effect by limiting the amount of food available to animals, says Roy - meaning smaller animals that need less food have a head start.

But Roy believes another factor threatens the world's most impressive animals. "Global warming may reinforce this trend towards smaller sizes through the temperature-size rule," he says.

Size rules

The temperature-size rule, also known as Bergmann's rule, says that species size increases with latitude: they tend to be smaller in the tropics, and larger closer to the poles. Bergmann's rule is debated, but one explanation for it is that larger animals have a lower surface-area-to-volume ratio, allowing them to retain more heat and fare better in cooler climes.

Conversely, smaller species radiate their heat more easily and so are better adapted to living in warm temperatures. There is also some experimental evidence that rearing animals in higher temperatures generally results in smaller individuals.

Roy has shown that the average body size of tiny ocean shrimps, known as ostracodes, gradually became bigger several million years ago. As the world cooled by 12°C during the Cenozoic era, fossil ostracodes got about 30 microns bigger for every degree of cooling (PNAS, DOI: 10.1073/pnas.0510550103). Although he does not yet have the data, he now says we should expect the opposite consequence from human-induced global warming.

Looking for the small

"It makes sense to be bigger when it's colder," says Wendy Foden, a biologist at the World Conservation Union who is studying the effects of climate change on species. "As the world gets warmer, the converse will happen, species will shrink."

Andy Purvis of Imperial College's Centre for Population Biology in the UK agrees. "There are bound to be exceptions - species that find energy more easy to come by in a warmer climate - but generally I would expect species to shrink because they'll be coping with sudden changes in their environment and dying younger."

In 2005, Purvis was part of a study showing that species that are threatened with extinction are on average one order of magnitude larger than those that are not (Science, DOI: 10.1126/science.1116030).

To Foden and Purvis' knowledge, ecologists have yet to identify a species that has shrunk as a result of global warming - though many have shrunk because of fishing, hunting, and the degradation of their habitats. Because the shrinking is an evolutionary response, it will take a long time for it to be noticeable.

Foden says the most likely place to spot it would be in an environment that has already experienced significant warming, and among species with short generation times. So it might be time to start scrutinising animals in the Arctic.

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